Booth for electrostatic powder painting with contact type electric field curtain

ABSTRACT

A booth in which electrically charged particulate material is sprayed onto a workpiece having an opposite charge, so that the particles are electrostatically attracted to the workpiece, has all of its walls that confront the workpiece made of electrical insulating material. A grid-like arrangement of parallel, spaced apart electrodes, insulated from each other, extends across the entire area of every wall, parallel to a surface of the wall and in intimate juxtaposition thereto. Each electrode is connected with one terminal of an alternating high voltage source, every electrode with a different terminal than each of the electrodes laterally adjacent to it, to produce a constantly varying field that electrodynamically repels particles from the wall.

United States Patent [1 1 Masuda 1 1 Apr. 2, 1974 1 BOOTH FOR ELECTROSTATIC POWDER PAINTING WITH CONTACT TYPE ELECTRIC FIELD CURTAIN [21] Appl. No.: 292,160

[30] Foreign Application Priority Data Oct. 6, 1971 Japan 46-78470 [52] US. Cl. 317/3, 98/115 SB, 134/1 [51] Int. Cl B08b 7/00 [58] Field of Search 98/115 SB; 15/1.5; 134/1;

118/628, DIG. 7

OTHER PUBLICATIONS IBM Technical Disclosure Bulletin Vol. 7, No. 3-Aug. 1964, p. 181, Removing Impurities From Surfaces.

Landry ct a1. 134/1 Dingman 417/48 Primary ExaminerWilliam M. Shoop, .lr. Assistant Examinerl-larry E. Moose, Jr.

[57] ABSTRACT A booth in which electrically charged particulate material is sprayed onto a workpiece having an opposite charge, so that the particles are electrostatically attracted to the workpiece, has all of its walls that confront the workpiece made of electrical insulating material. A grid-like arrangement of parallel, spaced apart electrodes, insulated from each other, extends across the entire area of every wall, parallel to a surface of the wall and in intimate juxtaposition thereto. Each electrode is connected with one terminal of an alternating high voltage source, every electrode with a different terminal than each of the electrodes laterally adjacent to it, to produce a constantly varying field that electrodynamically repels particles from the wall.

4 Claims, 11 Drawing Figures BOOTH FOR ELECTROSTATIC POWDER PAINTING WITH CONTACT TYPE ELECTRIC FIELD CURTAIN This invention relates to a booth for electrostatic powder painting having a device for removing powder particles from the wall of the booth with a contact type electric field curtain.

Hitherto in the actual practice of electrostatic painting with powder, various methods have been employed. For example, powder for painting is dispersedly discharge into the space in front of the object to be painted and is electrically charged by means of a gun supplied with a d.c. high voltage and electrostatic force drives the powder onto the surface of the object to be adhered thereon, after which for example, by heating, said powder is adhesively attached to said surface. In this instance, since a part of the powder particles dispersed in the space do not attach tothe object to be painted, a booth is provided to surround the space in which the painting operation is performed and in which the object to be painted is placed, and the floating particles that are not arrested to the object are carried by air through a hood arranged in the booth are collected in a suitable dust collector such as a cyclone or bag filter.

However, in this instance, the floating powder particles, having been electrified, tend to become attached to the inner wall of the booth due to the influence of an electric image; and further, since such particles are of a substance such as plastic, resin or the like having high electric resistance, once attached to the wall, they tend to hold an electric charge for a longtime. Consequently, as the painting operation proceeds, it has been usual that powder particles accumulate on the inner wall of the booth, the inner wall of the hood, and, in some cases, the inner wall of a duct leading from said hood to the dust collector, and on the inner wall of a hopper beneath the hood. In such cases it is difficult to strip off powder particles accumulatingly deposited on the walls merely by mechanical hammering.

Therefore when kind or color of powder particles has been changed in the course of a painting operation, such particles of different kinds or colors are mixedly deposited on said inner walls; and as these mixed powders are collected in a mass, it becomes difficult to separately sort and collect them. This spoils the utility value of the powder resulting in considerable economic loss.

The object of the present invention is to eliminate such difficulties by preventing painting powder from settling on said inner walls of the booth, hood, hopper, cyclone or bag filter. This is done by continually or in termittently removing whatever sort of painting powder attaches to the walls using electrical means whereby any powder clinging to the inner wall of the booth is completely swept off. If kind or color of powder has been changed in the course of a painting operation, the present invention nevertheless makes it possible to separately sort and collect the different powder particles.

The object of the present invention is achieved by providing contacting type electric field curtain apparatus having a powerful electrodynamic function for driving deposited powder off the inner wall of the booth, hood, duct, hopper and various dust collectors and removers.

The contact type electric field curtain apparatus just referred to is a device that functions to drive off what ever sort of powder is settled on the wall by producing a constantly changing electric field that employs an electrodynamic principle.

The basic construction and operation of that apparatus will become more apparent as the description proceeds with reference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic view of stationary wave contact type electric field curtain apparatus according to the present invention.

FIG. 2 is a similar view of traveling wave contact type electric field curtain apparatus.

FIG. 3 is a front view of one example of stationary wave contact type electric field curtain apparatus that can be use, in the present invention.

FIG. 4 is a section view taken on the plane of the line IV-IV in FIG. 3.

FIG. 5 is a front view of another example of stationary wave contact type electric field curtain apparatus that can be used in the present invention.

FIG. 6 is a section view taken on the plane of the line VI-VI in FIG. 5.

FIG. 7 is a front view of an example of a traveling wave contact type electric field curtain apparatus.

FIG. 8 is a section view taken on the plane of the line VIII-VIII in FIG. 7.

FIG. 9 is a front view of still another example of electric field curtain apparatus.

FIG. 10 is a section view taken on the plane of the line XX in FIG. 9.

FIG. 11 is a diagrammatic view of one example of a novel booth for electrostatic powder painting according to the present invention.

In FIG. 1, numeral 1 designates an insulator or semiconductor body I (referred to as an exciter) placed in close or contacting relation to a number of electrodes 2, 3,2, 3' that are arranged in parallel and insulated relation to one another, these electrodes being alternately connected to conductors 4 and 5, respectively in two groups, and said leads 4 and 5 being further connected through leads 6, 7 respectively to the secondary terminals A, B of a single-phase transformer 8. When an ac voltage is applied thereto, there is produced between adjacent electrodes a series of stationary wave constantly varying alternating electric fields 10 having electric force lines 9 as shown by dotted lines 9. Some of the lines of force, as indicated at 11, penetrate through the exciter and enter into the space 12 at the side of the exciter remote from the electrodes. Now if powder 14 of any kind deposits on the surface 13 of the exciter, that is remote from the electrodes, then, because of contact electrification between the powder and the exciter either one is positively charged and the other is negatively charged.

Hence the stationary wave constantly varying electric field curtain generally exerts a strong electrodynamical repulsive action upon the charged powder particles, whether they be positively or negatively charged, to drive in the direction of arrow 15, that is in the direction away from the series of electrodes 2, 3, 2, 3' and thus said power particles 14 are repelled in the direction 15 by action of the constantly varying altemating electric field curtain penetrated into said area 12 whereby the particles are removed from the surface 13 of the exciter and fall down by gravity in the direction of arrow 16.

Thus there is provided a combination of an exciter to create a contact charge by contact with object powders and a constantly varying alternating electric curtain functioning to repel the charged powders. The inventor named the apparatus that has repelling and driving action to powders as a contact type electric field curtain, and also named apparatus based on this as contact type electric field curtain apparatus. Addition to the stationary wave constantly varying alternating electric field curtain produced by applying a single-phase alternating voltage as shown in FIG. 1 it is also possible to produce a traveling wave constantly varying electric curtain field formed by applying a poly-phase alternating voltage in phase order to the corresponding electrodes. In the latter case, besides electrodynamic repulsion there is generated an electrodynamic propelling action that to forcibly transports powder particles, whether they be positively or negatively charged, in the direction that the traveling wave is oriented.

FIG. 2 shows one example of such travelling wave apparatus with three-phase alternating voltage used as an electric source.

An exciter l is placed in close or contacting relation to a number of electrodes 17, 18, 19, 17', 18, 19 arranged in parallel and in insulated relation to one another. The electrodes are divided into three groups by connecting every third electrode to one of the conductors 20, 21, 22 said conductors are further connected respectively in phase order to the corresponding secondary terminals U,V,W of-three-phase transformer 26, in either the primary or the secondary side thereof, the neutral point of Y connection being earthed, whereby between adjacent electrodes l7, 18, 19, 17, 18, 19'

there is produced a traveling constantly varying alternating electric field 29 represented by electric force lines 28 which field advances along the electrodes in a direction 27 a part of the lines of force pass through the exciter l and come into the area 12 at the side of the exciter remote from the electrodes. In contacting the surface 13 of the exciter 1, deposited charged powder particles 14 are strongly repelled with electrodynamic force in the direction of arrow 15 and are concurrently driven in the direction of arrow 27. As a result, the deposit of powder particles 14 is forcibly driven off from the surface 13 of the exciter l and is transported in the direction of arrow 27. Of the contact type electric field curtains, utilizing the traveling wave constantly alternating electric field is called as traveling wave contact type constantly varying electric field curtain.

In general all devices that produce the stationary wave or traveling wave contact type electric field curtain can be regarded as electric field curtain apparatus. Therefore, here the contact type electric field curtain apparatus can be incorporated in a variety of devices having (for ex. a bag filter air-permeable contact type.

electric field curtain apparatus applied to its cloth, an air slide conveyor having an air-permeable plate is comprising such a said unit, unit, ora duct or chute constituted with a cylinder shaped contact type electric field curtain) In these instances, if deposited powder particles have already been sufficiently electrified by a previous charging that adapts them to be used for painting, the operation of such contact type electric field curtain apparatus will have a strange electrodynamic effect. I

Accordingly when such stationary wave or traveling wave contact type electric field curtain apparatus is arranged to the inside wall of a booth, hood, duct, hopper or any kind of dust collector or dust removing device on which spreading power settles, the powder can be continually or intermittently removed therefrom solely by electrical means and without need for mechanical removal apparatus.

In this connection, the electrodes in FIG. I and FIG. 2 not need be of metal but instead any electrically conductive painting film, plastic, fiber, carbonic fiber or any other conductive material may be used. Also the electrodes can be in the shape of a bar, panel, plate, net or any other suitable shape. Further, the electrodes may be placed not only in close or contacting relation to one side of the exciter, as shown in FIG. 1 and FIG. 2, but could also be placed on the opposite sides of the exciter or could be embedded in the exciter.

The exciter can be of any organic material such as plastic, bakelite, or ebonite, or of any inorganic material such as glass, cement, asbestos. In this instance, as for fire protection, incombustible or noninflammable material is preferably used for the exciter. Needless to say, the exciter may be in the form of a plate, sheet, filament, net, cloth, fiberous layer, porous plate, thread, tape or tube or any other suitable shape.

As applicable voltage, either single-phase or polyphase alternating voltage may be used and as the case may be, a dc. voltage may be superposed or an impulse voltage may be applied or superposed. Also as it may be necessary to remove an electric charge from the stripped powder layer and to prevent its particles from resettling, any unelectrifying device utilizing a.c. or d.c. corona discharge, or unelectrifying device utilizing radiation or any other suitable means, may be arranged in the booth, hood, duct, dust collector, hopper or in any other suitable location to function in cooperation with a contact type electric field curtain.

Now a contact type electric field curtain apparatus to v be used for a novel booth according to the present invention will be specifically described with reference to the drawings.

FIG. 3 is a front view of a unit of contact type electric field curtain apparatus to be used in a novel booth according to the present invention in which a number of sheet electrodes are embedded in a plate exciter made of incombustible synthetic resin such as teflon, vinyliden fluoride, or vinyl fluoride, to produce a stationary wave contact type electric field curtain as illustrated in FIG. 1. In FIG. 3 and FIG. 4, in a plate exciter 1 of about 4mm thick, there are embedded a number of aluminum sheet electrodes 2, 3, 2, 3'....., each of about 10mm wide and 0.1mm thick and arranged in parallel to one another with equal spaces of 10mm between them to produce a stationary wave electric field curtain.

The electrodes are alternately connected to conductors 3,4 likewise embedded, and via terminals 31, 32 and 31', 32', are connected in series or parallel to an adjacent similar unit and further connected to a singlephase a.c. power source (not shown). Thus, on the opposite sidesof the exciter 1 there is formed a stationary wave contact type electric field curtain like that shown in FIG. 1.

FIG. 5 is a front view of another contact type electric field curtain apparatus unit that can be used for a novel booth. Therein, on the surface of a plate exciter, electric wires sheathed with incombustible resin are arranged. In FIG. 5 and FIG. 6, the plate exciter l which is about 3mm thick, has in its opposite sides recesses 33, 34, 33', 34' and 35, 36, 36' of which alternate recesses differ in depth. On the face of the exciter, two sheathed wires 37, 38, each having 3mm outer diameter, are arranged in 15mm spaced apart relation to each other the respective recesses and have their opposite ends connected to adjacent similar units (not shown) respectively through terminals 31, 32 and 31, 32 leading to a single-phase a.c. power source (not shown) whereby a stationary wave contact type electric field as in FIG. 1 is formed on the surface of the exciter 1.

FIG. 7 is a front view of a unit generally like that of FIG. 3 and FIG. 4 but adapted to create a traveling wave contact type electric field curtain as shown in FIG. 2 in the opposite sides of the exciter, by reason of the application of a three-phase a.c. voltage instead of single-phase a.c. voltage. Every third one of the sheet aluminum electrodes 17, 18, 19, 17', 18', 19' enbedded in the exciter is connected to one of the conductors 20, 21, 22 which are likewise embedded and which are connected through terminals 39, 40, 41, 39', 40', 41 to an adjacent unit and are further connected respectively in phase order to corresponding output terminals U, V, W of a three-phase a.c. current source (not shown) whereby there is formed a downwardly traveling wave contact type electric field curtain on the opposite sides of the exciter 1.

, FIG. 9 is a front view of a unit in which a traveling wave contact type electric field curtain like that shown in FIG. 2 is formed on the opposite sides of an exciter by applying three-phase a.c. voltage. The plate exciter 1 like that in FIG. 5 and FIG. 6, has in its opposite sides recesses 42, 43, 44, 42, 43, 44 and 45, 46, 47, 45, 46, 47 three progressively different depths. As shown, three sheathed wires 48, 49, 50 extend in parallel and equally spaced relation to one another on the surface of the exciter l and through corresponding recesses; and at their opposite ends they are connected to terminals 39, 40, 41 and 39', 40, 41' respectively for connection to adjacent units and to be further connected to output terminals of a three-phase a.c. power source (not shown). There is thus produced on the surface of the exciter a traveling wave contact type electric field curtain which advances downwardly.

FIG. 11 is an example of a novel booth for effecting electrostatic painting with powder, incorporating a stationary wave or traveling wave contact type electric field curtain apparatus unit as shown in FIG. 3 to FIG. 10.

The booth 52 has an opening 51 through which a powder spray nozzle or gun 61 can be directed towards a workpiece 58 in the booth. In the upper interior 53 of the booth, an earthed rail 54 is provided, in the grooves 55 of which a pair of rollers 56 are slidable. The workpiece 58 to be painted is suspended with a metal suspender 57 from said rollers. As the workpiece moves along the rails, painting is effectedwith powder particles 62 blown from the nozzle 61 and charged with negative high voltage from a d.c. negative high voltage source 59 through a cable 60 connected to the nozzle. Then the dispersingly discharged powder particles 63 are drawn by air from the inside 53 of booth 63 through hood 64 and duct 65 to a cyclone 67 that is provided with a suction fan 66. In the cyclone the particles are collected and separated from the air. Clean air is discharged upward through duct 68. The collected powder particles for painting are delivered through an airtight valve 69 onto an air-slide conveyor 70 are transported by it to a collector pipe 71. Then by means of a selector valve 72, the particles are sorted (in this case, it is assumed they are of two kinds) and they are returned to respective tanks 73, 74 from which they can be withdrawn and fed to the nozzle 61 for re-use. Also particles slipping down from the deposit on the inner wall of the booth 52 and particles 96 settling by gravity directly from the interior of the booth 52 are together carried by another air slide conveyor 97 and are returned through collector pipe 71 to the powder tank 73 or 74.

The returned powder particles in the tanks are supplied through another selector valve 75 and outlet pipe 76 to an ejector 77. Then by action of pressure air supplied from piping 78, the particles are delivered through a flexible pipe 79 to the gun 61 and are again ejected therefrom, electrically charged.

As already stated, to remove charged powder particles deposited on the wall, by means of the electrodynamic driving action of the contact type electric field curtain, a suitable contact type electric field curtain apparatus unit as shown in FIG. 3 to FIG. 10 or any other contact type electric field curtain may be used. In the embodiment shown in FIG. 11, there are traveling wave contact type electric field curtain apparatus units 80, 81, 82 as shown in FIG. 9 and FIG. 10.

Also, units 83, 84 of the same type are arranged on the inner wall of the hood 64.

The units are connected, through assigned triple terminals, in phase order to secondary tenninals U, V, W of a three-phase transformer 26 similar to that shown in FIG. 2, with a switch 85 on its primary side. Thereby on the surface of each unit 84, a traveling contact type electric field curtain is formed whereby as stated before, the inner wall of the booth and hood are kept completely clear of deposits of powder particles. Switch may be either kept closed or intermittently closed. In this case, with the provision of a d.c. source, the secondary neutral point 86 of three-phase transformer 26 is given a negative voltage against earth whereby each electrode of the contact type electric curtain apparatus units 80 84 is kept to a high negative voltage, that is, maintained at the same polarity as the nozzle 61 relative to the d.c. earthed workpiece 58.

As a result negatively charged particles 63 floating in the interior 53 of the booth are influenced by an electric force that drives them from the surface of the booth wall to the workpiece 58, thus further preventing deposits of the powder particles on the wall surface. On the inner wall of duct 65, stationary wave contact type electric field curtain apparatus units 88, 89, of the type shown in FIG. 3 and FIG. 4, are arranged, each being connected through two terminals to the secondary terminals X, Y of the three-phase transformer 26 thus to form a stationary wave contact type electric field curtain whereby deposition of powder particles on the inner wall of the duct is completely prevented.

In this instance, needless to say that instead of arranging the contact type electric field curtain units 88, 89 at the local positions, it is possible to form the duct 65 as a cylindrical stationary or traveling wave contact type electric field curtain unit.

Pressure air is forced from tubes 90, 91 the respective air slide system conveyors 70, 97 into their lower chambers 92, 93 respectively, and thereabove are fiber layers or porous plates 94, 95 made of suitable insulating material which serve as exciters and in each of which three sets of plate electrodes like those shown in FIG. 6 and FIG. 7 are embedded. Those electrodes are connected through a corresponding three terminals in phase order to the respective secondary termina'ls U, V, W of the three-phase transformer, whereby above the respective plates 94, 95 there is produced a traveling wave contact type electric field curtain oriented to the right which transports electrically charged powder particles without depositing them on said conveyors 70 97.

In this connection, instead of the air slide system conveyor, a chute, screw conveyor, chain conveyor or any other transportation device may be used. And said contact type electric field curtain apparatus unit may comprise the dust depositing face of the conveyor or the conveyor casing itself may be made as a cylindrical or other suitably shaped stationary wave or traveling wave contact type electric field curtain apparatus. Also the inner wall of the cyclone 67 may be lined with a suitably shaped contact type electric field curtain, or instead of the cyclone 67, a bag filter may be used comprised of air-permeable cloth or having a fiber layer which itself constitutes an exciter of a contact type electric field curtain apparatus.

As specifically described above, the novel device of the present invention is enabled to effectively prevent painting powders from settling on the inner wall of the booth, hood, duct, conveyor and dust collector with solely electrical means and without the need for any mechanical means. Therefore when it is necessary to change the kind or color of powder, it is possible at any time to remove and collect previously used powder so that there will not be any mixture of power in painting.

A booth according to the present invention may be used not only in electrostatic powder painting but also for electrostatic powder coating on the inside of pipe, electrostatic printing, electrostatic coloring, electrostatic pile planting and for any other operation that involves imparting an electrostatic charge to any powdery or fiberous object.

What is claimed is:

1. A booth for electrostatic spray coating, having a zone at its interior at which a workpiece can be located, said booth being adapted for use with apparatus of the type comprising a nozzle that is directed toward said zone and from which particulate material to be coated onto theworkpiece can issue, and means for impressing an electrical charge of one polarity upon particles of said material before they issue from the nozzle and for impressing upon the workpiece an opposite charge that tends to attract the particles to it electrostatically, said booth being characterized by:

A. substantially all walls of the booth that are in confronting relation to said zone being of a material that is substantially an electrical nonconductor;

B. a plurality of elongated electrodes distributed over substantially the entire area of a surface of each of said walls, said electrodes extending parallel to said surface and beingin intimate juxtaposition thereto, and being in electrically insulated relation to one another, and substantial portions of the electrodes being in closely spaced parallel relation to one another and arranged alternately with one another in a regular sequence;

C. means for connecting each of said electrodes with only one of the plural terminals of different polarity of a source of high alternating voltage, each electrode being connected with a terminal of said source that is different from the terminal to which each of its adjacent electrodes is connected, so that the several electrodes cooperate to produce a constantly varying electrical field by which charged particles are repelled from the walls.

2. The spray booth of claim 1, further characterized D. there being at least three of said electrodes distributed over the area of the bottom wall portions of said booth and alternating with one another in a regular sequence thereacross; and

E. said means for connecting each of the last mentioned electrodes with a terminal of a high alternating voltage source providing for such connection of said electrodes with a multi-phase high alternating voltage source that there is a phase difference between every electrode and its laterally adjacent electrodes, so that the constantly varying electric field conjointly produced by all of said last mentioned electrodes has a wave-like variation by which charged particles are both repelled from the inner surface of said bottom wall portions and urged along the wall in one direction transverse to said parallel portions of said last mentioned electrodes.

3. The spray booth of claim 2 wherein all of said bottom wall portions are inclined to the horizontal and slope downwardly in said one direction so that the force of gravity tends to supplement the effect of the constantly varying electric field in transporting the particles in said one direction, further characterized by:

F. collecting means having an inlet adjacent to the lowest edge portions of said wall portions and into which particles can fall after being transported along said wall portions, to be available for re-use.

4. The spray booth of claim 3, wherein bottom wall portions of the spray booth are porous, further characterized by:

G. means for directing a flow of pressurized gas upwith particles entrained in such gas. 

1. A booth for electrostatic spray coating, having a zone at its interior at which a workpiece can be located, said booth being adapted for use with apparatus of the type comprising a nozzle that is directed toward said zone and from which particulate material to be coated onto the workpiece can issue, and means for impressing an electrical charge of one polarity upon particles of said material before they issue from the nozzle and for impressing upon the workpiece an opposite charge that tends to attract the particles to it electrostatically, said booth being characterized by: A. substantially all walls of the booth that are in confronting relation to said zone being of a material that is substantially an electrical nonconductor; B. a plurality of elongated electrodes distributed over substantially the entire area of a surface of each of said walls, said electrodes extending parallel to said surface and being in intimate juxtaposition thereto, and being in electrically insulated relation to one another, and substantial portions of the electrodes being in closely spaced parallel relation to one another and arranged alternately with one another in a regular sequence; C. means for connecting each of said electrodes with only one of the plural terminals of different polarity of a source of high alternating voltage, each electrode being connected with a terminal of said source that is different from the terminal to which each of its adjacent electrodes is connected, so that the several electrodes cooperate to produce a constantly varying electrical field by which charged particles are repelled from the walls.
 2. The spray booth of claim 1, further characterized by: D. there being at least three of said electrodes distributed over the area of the bottom wall portions of said booth and alternating with one another in a regular sequence thereacross; and E. said means for connecting each of the last mentioned electrodes with a terminal of a high alternating voltage source providing for such connection of said electrodes with a multi-phase high alternating voltage source that there is a phase difference between every electrode and its laterally adjacent electrodes, so that the constantly varying electric field conjointly produced by all of said last mentioned electrodes has a wave-like variation by which charged particles are both repelled from the inner surface of said bottom wall portions and urged along the wall in one direction transverse to said parallel portions of said last mentioned electrodes.
 3. The spray booth of claim 2 wherein all of said bottom wall portions are inclined to the horizontal and slope downwardly in said one direction so that the force of gravity tends to supplement the effect of the constantly varying electric field in transporting the particles in said one direction, further characterized by: F. collecting means having an inlet adjacent to the lowest edge portions of said wall portions and into which particles can fall after being transported along said wall portions, to be available for re-use.
 4. The spray booth of claim 3, wherein bottom wall portions of the spray booth are porous, further characterized by: G. means for directing a flow of pressurized gas upwardly through said porous bottom wall portions and into the interior of the spray booth; H. filter means external to the spray booth, for filtering particles of said material out of gas in which they are entrained; and I. passage defining means having an inlet communicated with the interior of the spray booth near the top thereof and having an outlet opening to said filter means, for conducting to the filter means gas that has flowed upwardly through the spray booth from said porous bottom wall portions, together with particles entrained in such gas. 